Periodic Table, The: Past, Present, And Future

by Geoffrey Rayner-canham;

  Silver is the middle element of the Group IB series, yet in many of its physical properties it exhibits extreme values rather than values which fall between those of copper and gold, e.g. melting point (min.), boiling point (min.), thermal conductivity (max.), etc. It exhibits a steel grey colour similar to that of a majority of metals similar to that of the majority of metals rather than a colour similar to that of copper or gold.

  To extend the argument to the chemical properties, as shown in Table 8.8: for copper, the +2 oxidation state dominates; for silver, the +1 state; and for gold, the +3 state.

  Table 8.8 Comparative species for the [Cu–Ag–Au] ions under oxidizing conditions

  Table 8.9 Comparative species for the [Fe–Co–Ni–Cu] tetrad under mild oxidizing conditions

  In fact, in terms of its coordination chemistry, copper fits better with the later 3d elements. For example, in the species across the oxidizing pH range, we see a strong similarity (Table 8.9).

  The dominance of the +1 d10 state for the normal aqueous chemistry of silver [31] make it more appropriately considered as a main group metal. As to be discussed in Chapter 10, silver is the “classic” example of the “knight’s move” linkage, showing a startling similarity to thallium. A unique parallel is that they are the only two metal ions to form brick-red insoluble chromates, Ag2CrO4 and Tl2CrO4. Under oxidizing conditions, we have the parallel shown in Table 8.10.

  Gold is a very different element to that of copper and silver. In fact, it has been referred to as the gold anomaly [32]. This anomaly is largely ascribed to the importance of the relativistic effect, as mentioned in Chapter 2. Just as there are unusual species linking silver(I) and thallium(I), so gold(V) shows a strong resemblance to platinum(V). A good example is the pair of compounds: [O2]+[PtF6]− and [O2]+[AuF6]−.

  Table 8.10 Comparative species for the [Ag–Tl] diad under oxidizing conditions

  Table 8.11 Comparative species for the [Au–Pt] diad under oxidizing conditions

  The species for platinum and gold can be compared under oxidizing conditions. Allowing for the fact that +4 is the dominant oxidation state of platinum, and six, its common coordination number, there is again a much closer parallel of gold with platinum than with silver and copper (Table 8.11).

  Gold is actually the easiest one of this Group to assign to a cluster: that is, to the platinum metals. It is the platinum metals plus gold that are the only metals found almost entirely in their elemental state [33]. In fact, gold is sometimes found in nature as an alloy with platinum and palladium [34]. The acceptance of this “cluster” was cited by Barnes et al. [35]:

  Au is strictly speaking not a platinum-group element but it is a noble metal and will be included with the PGE’s in this paper.

  This Author supports the addition of gold to the platinum metals with the designation of “noble metals” (see Chapter 5).

  Geochemists often extend this cluster to include rhenium. These eight elements, [Os–Ir–Ru–Rh–Pt–Pd–Re–Au], are referred to as the highly siderophile elements (HSE), that is, elements found throughout the solar system most commonly in elemental form [36].

  A Hybrid Solution

  So, is there a classification of the transition elements that better reflect the linkages? Obviously, we cannot satisfy all the many similarities, but the argument is made here that the best fit is not accomplished by either the Group or the Period approach. Instead, a hybrid combination generates the clusters of elements that have similarities worthiest of highlighting (Figure 8.5). As one example of the allegiance of titanium to the [Zr–Hf–Nb–Ta] tetrad is that all five of the elements form trisulfides of the form MS3 [37].

  Subsequent to deducing the splitting of allegiances for titanium and manganese, Leal and Restrepo have highlighted the same divisions in their “ordered hypograph” [38].

  Figure 8.5 A hybrid approach to transition metal classification.

  To review, the most logical clustering is listed in the following, showing the “secondary allegiances” of titanium and of manganese:

  •The [Ti–Zr–Hf–Nb–Ta] pentad whose simple chemistry is dominated by insoluble oxides.

  •The [V–Cr–Mn] triad that exhibits soluble, oxidizing, isoelectronic tetraoxo-anions plus a stable +3 oxidation state, to which Ti can be appended for other aspects of common chemistry.

  •The [Fe–Co–Ni–Cu] tetrad for which the +2 aqueous ion is a major component of simple chemistry, to which Mn can be appended for some commonalities.

  •The [Mo–W–Tc–Re] tetrad for which nonoxidizing soluble valence-isoelectronic tetraoxo-anions exist.

  •The [Ru–Os–Rh–Ir–Pd–Pt–Au] heptad that can be defined as the “noble metals.”

  •The [Ag–Tl] diad that reminds us to “think outside the (transition metal) box.”

  Commentary

  Chemists like smooth patterns — continuities — systematic trends. Sorry, it doesn’t happen with the transition metals! Instead, each of the transition metals flaunts its individuality and refuses to fit neatly into a specific category. The 3d metals are fractured into two halves; zirconium and hafnium behave like twins; silver seems more at home outside the transition metals; gold nestles up to the platinum metals; and titanium and manganese have split allegiances between their Period and their Group. Never let it be said that transition metal chemistry is predictable and boring!

  References

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  Chapter 9

  Group (n) and Group (n + 10) Relationships

  The early Periodic Tables displayed an eight-Group system. Though we now use an 18-Group array, the old versions were based on evidence of similarities between the elements in what we now label the Group (n) and the corresponding elements of Group (n + 10). In this chapter, these similarities will be explored in depth. It is shown that such linkages are not limited to the top members of each Group as earlier discussions have emphasized.

  In Chapter 6, the term pseudo-isoelectronic was introduced. This term described a subset of valence-isoelectronic linking a main group element and a transition element. Such electron configurations differ specifically by d10 or f14d10. For example, the Mg2+ ion and the Ca2+ ion are valence-isoelectronic, but they each differ from the Zn2+ ion by the 3d10 electrons. Therefore, the Zn2+ ion is pseudo-isoelectronic of the two main group ions. This chapter specifically focuses upon these pseudo-isoelectronic relationships.

  Going Back to the Past

  It was Newlands who first proposed that the chemical elements could be organized according to the “Law of Octaves” [1, 2]. The later Periodic Tables produced by Mendeleev also utilized an eight-column table. This was done even though it meant that the [Fe–Ni–Co]; [Ru–Pd–Rh]; and [Os–Pt–Ir] series were squished into the single Group VIII (see Figure 9.1) [3]. Nevertheless, a key point of the table occupancy was the element similarities within each of the Groups. As an example, in Group V, there were similarities among all eight elements: nitrogen, phosphorus, vanadium, arsenic, niobium, antimony, tantalum, and bismuth.

  Figure 9.1 A simplified version of one of Mendeléev’s designs of the Periodic Table.

  The Rise of the Long Form of the Periodic Table

  In 1893, Rang devised one of the first long-form Periodic Tables [4]. He numbered the columns from I through VIII then I through VII (as the noble gases were then unknown). With duplicate numbering for Groups I through VII, it seems to have been Deming in 1923 who first used “A” and “B” designations in a pedagogical context to clarify which groups were which [5]. This was the system adopted by the American Chemical Society (ACS), such that what we now call Group 3 was labeled as “IIIB” while Group 13 was labeled “IIIA.” However, the International Union of Pure and Applied Chemistry (IUPAC) adopted a system whereby what we now call Group 3 was labeled as “IIIA” and Group 13 was labeled “IIIB” [6].

  It was to resolve this confusion, that the 1 through 18 notations were proposed by IUPAC in the 1980s and subsequently adopted worldwide [7]. A disadvantage has been that the similarities that caused Mendeleev and others to put, for example, silicon and titanium in the same Group, became largely forgotten or overlooked. Only Sanderson in 1954 bravely continued to expound the pedagogical benefits of the eight-column table [8]. For example, in his opinion, that it made much more sense, in teaching general chemistry to have a unitary Group 3 consisting of B–Al–Sc–Ga–Y– In–La–Tl–Ac, all of which have a common oxidation state of +3. The common oxidation states of the other Groups: 0 to VII can be readily identified in a similar manner.

  The Rediscovery of the A and B Links

  It was Laing who, in 1989, first reminded the modern generations of chemists of these similarities [9, 10]. He noted the resemblances between silicon and titanium compounds (such as the pair SiCl4 and TiCl4); phosphorus and vanadium compounds (such as POCl3 and VOCl3); sulfur and chromium polyatomic ions (such as and ); and chlorine and manganese compounds (such as Cl2O7 and Mn2O7). To emphasize the linkage, Laing proposed that the element “boxes” of lithium to fluorine and sodium to chlorine be repeated above the corresponding transition metal column.

  Rich [11] proposed a modification to Laing’s diagram: that oxygen and fluorine be deleted from the duplication as there are no similarities between those elements and the corresponding transition metals of chromium and manganese. In fact, there is little similarity between any of the 2nd Period main group elements and the corresponding d-block elements. It is of note that all of Laing’s examples compare 3rd Period main group elements with the 4th Period d-group elements. Thus, the segment of the Periodic Table in the following, derived from Rich and Lang, simply show the addition of the respective Period 3 main group elements to the top of the respective transition metal columns (Figure 9.2).

  Laing’s study focused on the formula similarities of the 3rd Period main group elements with the corresponding Groups of the transition series. However, Mingos showed that such parallels in formula existed in other (n) and (n + 10) pairs [12]. Here, a wide exploration of such connections will be made.

  Figure 9.2 A segment of the Periodic Table showing the proposed additional 3rd Period members of Groups 3–7 and 12.

  Definition of the Group (n) and Group (n + 10) Relationship

  The linkage in chemical formulas and chemical behavior between each Group (n) member and the corresponding Group (n + 10) member is quite specific. The relationship is between compounds and polyatomic ions of the highest oxidation state of the main group elements and those of the same oxidation state of the matching transition elements. A general definition is [13]:

  The (n) and (n + 10) relationship identifies some similarities in some of Group (n) members with those of the corresponding Group (n + 10). This resemblance is usually in the highest oxidation state. Such similarities can be in chemical formulas and structures of compounds and polyatomic ions, and of their aqueous behavior.

  The (n) and (n + 10) linkage for highest oxi
dation states comes about through electronic structural similarities. That is, the (n) element in its highest oxidation state has a noble gas electron configuration while the corresponding (n + 10) element in its highest oxidation state has, in addition, a filled d10 set. For the elements lower in the respective groups, there is also a filled f14 electron set. As the metal is in a high oxidation state, the bonding in each compound is predominantly covalent. As examples, Table 9.1 shows oxo-anions of the 4th Period of Group 5, Group 6, and Group 7, together with the corresponding pseudo-isoelectronic oxo-anions of the 3rd Period and 4th Period of Group 15, Group 16, and Group 17.

  Table 9.1 Similarities in formula of some oxo-anions to illustrate the (n) and (n + 10) relationship

  Group 3 and Group 13

  It was Rang in 1893 who seems to have been the first, on the basis of chemical similarity, to place boron and aluminum in Group 3 (see Figure 9.3) [4].

  Such an assignment seems to have been forgotten until more recent times. Greenwood and Earnshaw [14] have discussed the way in which aluminum can be considered as belonging to Group 3 as much as to Group 13 (Figure 9.4), particularly in its physical properties. Habashi has suggested that there are so many similarities between aluminum and scandium that aluminum’s place in the Periodic Table should actually be shifted to Group 3 [15].

  Figure 9.3 The first section of Rang’s Periodic Table showing the location of boron and aluminum (from Ref. [4]).

  Figure 9.4 Members of Group 3 and Group 13.